1. Field of the Invention
The present invention relates to a signal processing method and apparatus for nuclear spectrometers, by the use of which a signal processing (amplitude analyser) system of small differential non-linearity and of theoretically possible highest operating rate with optimal energy resolution can be realized, which system is independent from the input intensity.
2. Description of the Prior Art
As known, up till now solely various analogous noise filtering methods have been used for improving for energy resolution (signal-to-noise ratio) of nuclear spectrometers.
The filtered pulses are sized and divided into groups by means of so called multichannel amplitude analysers (see e.g. the instrument of type SILENA Cicero of the Italian firm Silena S.p.A). These signals are converted into digital form and stored in the appropriate channels (at memory addresses) in accordance with their values so that the memory contents are incremented.
The function of the noise filters is to improve the accuracy of the pulse amplitude measurement, as much as possible, these pulse amplitudes being proportional to the energy induced on the effect of the detected particle or quantum in the detector.
The type of the detector (ionization chamber, semiconductor detector, etc.) and the preamplifier determine the spectral distribution of the electronic noise appearing on the preamplifier output.
It has been shown that in a given system the signal-to-noise ratio can be improved only up to a certain theoretical limit by using a so called Cusp-filter. However, this filter can not be realized in the practice. The realizable and nowadays used filters are characterized by means of the so called Cusp-factor, which means the noise-to-signal ratio of the realized filter related to the Cusp-filter. This value lies between 1.016 and 1.3.
The time interval between the rising point of the signal to be measured and the moment of its measurement is known as peaking time. In case of a given type of filter, the optimal peaking time can always be found and set, with which the best signal-to-noise ratio can be achieved. The duration of this peaking time is determined by the ratio of the main noise components (series and parallel noises) of the detector preamplifier system.
The optimal peaking time varies between close limits in the cases of the known filters, their busy times, however, differ from each other in a high degree, and in accordance with this fact, their throughput rates are also different.
The reason for this fact lies in that the signal amplitudes gotten by using the known noise filtering methods are influenced even by the pulses preceeding the actual signal with a long period of time. This results in a deterioration of energy resolution, in the function of the intensity. The measure of the abovementioned deterioration may be decreased by using base-line restorers and by inserting a so called protection time.
The total busy time (T.sub.B) is the sum of the peaking time (T.sub.P) and the protection time (T.sub.PR): EQU T.sub.B =T.sub.P +T.sub.PR ( 1)
In the case of the mostly used semigaussian filter T.sub.PR =4 T.sub.P, for filters having a very good Cusp-factor, the parameters of which are the function of the time, the equation PR=2 T.sub.P is valid.
In the case of continuous radiations (e.g. radioactive sources, continuously induced X-ray fluorescency, etc.) the time intervals between the subsequent pulses correspond to the Poisson-distribution. Supposing a semigaussian filtering, the output intensity measurable distortionfree (R.sub.out) can be expressed in the function of the intensity to be measured (R), as follows: EQU R.sub.out =Re.sup.-(T.sbsp.B.sup.+T.sbsp.P.sup.)R ( 2)